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Photochemistry of Dibenzothiophene-Based Sulfilimines Vasumathi Desikan Iowa State University

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Photochemistry of Dibenzothiophene-Based Sulfilimines Vasumathi Desikan Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 2007 Photochemistry of dibenzothiophene-based sulfilimines Vasumathi Desikan Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Organic Chemistry Commons, and the Physical Chemistry Commons Recommended Citation Desikan, Vasumathi, "Photochemistry of dibenzothiophene-based sulfilimines" (2007). Retrospective Theses and Dissertations. 15627. https://lib.dr.iastate.edu/rtd/15627 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Photochemistry of dibenzothiophene-based sulfilimines by Vasumathi Desikan A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Organic Chemistry Program of Study Committee: William S. Jenks, Major Professor Richard Larock Nicola Pohl Gordon Miller Klaus Schmidt-Rohr Iowa State University Ames, Iowa 2007 Copyright © Vasumathi Desikan, 2007. All rights reserved. UMI Number: 3289435 UMI Microform 3289435 Copyright 2008 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, MI 48106-1346 ii TABLE OF CONTENTS ABSTRACT iv CHAPTER 1: GENERAL INTRODUCTION – Photochemical Generation of Nitrenes 1 1.1 Dissertation Organization 1 1.2 Objective 1 1.3 Nitrenes and their photochemical precursors 2 1.3.1 Photochemical generation of nitrenes from azides 7 1.3.1.2 Photochemistry of aroyl azides 7 1.3.1.2 Photochemistry of acetyl azides 13 1.3.1.3 Photochemistry of sulfonyl azides 14 1.3.2 Sulfilimines 20 1.3.2.1 Nomenclature and general properties 20 1.3.2.2 Photochemical generation of nitrenes 23 1.3.2.2a Photochemistry of N-acyl sulfilimines 24 1.3.2.2b Photochemistry of N-sulfonyl sulfilimines 27 1.4 References 28 CHAPTER 2: Photochemistry of Sulfilimine-based Nitrene Precursors: Generation of Both Singlet and Triplet Benzoylnitrene 36 2.1 Abstract 36 2.2 Introduction 37 2.3 Results 38 2.4 Discussion 56 2.5 Conclusions 64 2.6 Experimental Section 65 2.7 Acknowledgements 68 2.8 References 69 iii CHAPTER 3: Photochemistry of N-Acetyl, N-Trifluoroacetyl, N-Mesyl and N-Tosyl Dibenzothiophene Sulfilimines 75 3.1 Abstract 75 3.2 Introduction 76 3.3 Results and Discussion 79 3.4 Summary 118 3.5 Experimental Section 121 3.6 Acknowledgement 125 3.7 References 126 CHAPTER 4: GENERAL CONCLUSIONS 131 APPENDIX A: Supporting Information for Chapter 2 134 APPENDIX B: Supporting Information for Chapter 3 149 iv ABSTRACT Azides have been the most extensively investigated precursors for the photochemical generation of nitrenes. Direct spectroscopic studies as well as indirect product studies have provided adequate evidence for the intermediacy of nitrenes, in at least some azides. On the other hand, less is known on the photochemical decomposition of sulfilimines. A systematic study of the potential of sulfilimines as nitrene precursors is undertaken in the current work. N-Benzoyl, N-acetyl, N-tosyl, N-mesyl and N-trifluoroacetyl derivatives of dibenzothiophene sulfilimine have been synthesized and analyzed for their photochemical reactivities. The extended UV-Vis absorption of the dibenzothiophene moiety allows for long wavelength photolysis and consequently provides access to a wide variety of nitrenes including the parent N-H version. The syntheses of N-H and the N-formyl derivatives of dibenzothiophene sulfilimine have also been achieved. It has been established that benzoyl and acetylnitrenes have singlet ground states1,2 while tosylnitrene has a triplet ground state.3 Our studies using time-resolved IR spectroscopy provides direct evidence for singlet and triplet nitrenes from N-benzoyl and N- acetyl sulfilimine derivatives. Product studies indicate that decay via the singlet nitrene channel is the major process. N-tosyl and N-mesyl sulfilimine derivatives differ from the above two, in that, the primary decay channel is triplet nitrene capture. In the case of N- trifluoroacetyl analogue, we observed products due to both singlet and triplet nitrene intermediacy. The products observed depend on the ground state multiplicities of the corresponding nitrenes generated. Theoretical calculations were performed to estimate the singlet-triplet energy separations (∆ES-T) for mesyl and trifluoroacetylnitrene. Coupled- v cluster calculations predict a triplet ground state for mesylnitrene and a closely spaced singlet and triplet energies for trifluoroacetylnitrene. Hence, we observe products from both spin states in the case of trifluoroacetylnitrene while predominantly from the triplet in the case of mesylnitrene. The calculations support our inference from product studies. All the sulfilimines analyzed indicate a potential for S-N bond cleavage from the excited triplet state of the sulfilimine. This is in line with our proposed mechanism for the photochemical unimolecular S-O bond cleavage of dibenzothiophene-S-oxide leading to O(3P). References (1) Liu, J.; Mandel, S.; Hadad, C. M.; Platz, M. S. J. Org. Chem. 2004, 69, 8583- 8593. (2) Pritchina, E. A.; Gritsan, N. P.; Maltsev, A.; Bally, T.; Autrey, T.; Liu, Y.; Wang, Y.; Toscano, J. P. Phys. Chem. Chem. Phys. 2003, 5, 1010-1018. (3) Wasserman, E. Prog. Phys. Org. Chem 1971, 8, 319 - 336 1 CHAPTER 1 GENERAL INTRODUCTION - Photochemical Generation of Nitrenes 1.1 Dissertation Organization This dissertation consists of four chapters. Chapter 1 is a literature review of nitrenes generated photolytically from azides and sulfilimines. Chapters 2 and 3 discuss the photochemistry of dibenzothiophene-based sulfilimines as potential precursors to nitrenes. All time-resolved IR measurements in Chapters 2 and 3 were performed at Johns Hopkins University, while the author performed the entire product studies. Chapter 2 discusses the photochemistry of N-benzoyl dibenzothiophene sulfilimine and Chapter 3 focuses on the photochemistry of N-acetyl, N-trifluoroacetyl, N-mesyl and N-tosyl dibenzothiophene sulfilimines. Synthetic efforts towards N-H and N-formyl derivatives of sulfilimines are also addressed in Chapter 3. Chapter 4 is a general conclusion of the previous chapters. 1.2 Objective We have previously studied the photochemistry of dibenzothiophene-5-oxide and have provided supportive evidence for the formation of atomic oxygen, [O (3P)].1-5 The main goal of this work is to broaden the scope of the dibenzothiophene platform in generating reactive intermediates, more specifically, in generating nitrenes. Dibenzothiophene-based sulfilimines (nitrene precursors) were synthesized for this purpose and their photochemical behaviors were examined in the subsequent chapters. The spin states, plausible mechanistic details and the reactivity pattern of the nitrenes and other reactive intermediates generated from the sulfilimines are the main topics of exploration in Chapters 2 and 3. One other 2 objective of this work is to provide access to nitrenes that would otherwise require shorter wavelengths for their generation from other precursors. The sulfilimines analyzed in Chapter 3 address this topic considerably. 1.3 Nitrenes and their Photochemical Precursors Reactive intermediates have had special attention in the field of organic photochemistry. Intermediates, which are difficult to access under thermal conditions, can be relatively easily generated, detected and studied under photochemical conditions. One such intermediate is the nitrene (represented below) and its chemistry generated from azide sources has been studied extensively in the last several decades.6-10 N R The nitrene nitrogen possesses six valence electrons, which make it an electron- deficient species. However, if the neighboring R group on the nitrene contains a heteroatom, such as nitrogen, oxygen or sulfur, directly attached to the nitrene nitrogen, then the polarity of the nitrene is reversed and it behaves as a nucleophile. Nitrenes, analogous to carbenes, are known to have triplet ground states, although exceptions to this rule also exist in the literature.11-14 The electronic structure of the parent nitrene can be easily understood by elementary molecular orbital analysis. Figure 1 depicts `the MO diagram of the simplest nitrene, imidogen. Of the six valence electrons on NH, two are involved in the σ-bonding with the hydrogen (2σ) and two constitute the lone pair on nitrogen (3σ). The remaining two electrons are singly occupied in each of the degenerate non-bonding π-MOs arising from the 2px and 2py AO on nitrogen. Pauli’s exclusion 3 principle prevents the two electrons from being paired up in the same orbital and hence the ground state configuration of the nitrene is a triplet. 4!# "x "y 1s 2px 2py 2pz 3! 2s 2! 1s 1! N NH H AOs MOs AOs Figure 1. Molecular orbitals of imidogen (NH) The triplet constitutes the lowest electronic configuration, 3Σ-, while the 1Δ and 1Σ+ are the two immediate higher lying electronic states as indicated in Figure 2. The energy differences measured between the states have been determined spectroscopically.15-20 4 1"+ 1A' l o 60.6 m / l 1 a ! c k 1A' 1A" n i y g r 36.0 e n E 3"# 3A" Figure 2. Energies of different states of imidogen (NH) The singlet component 1Δ is comprised of the “closed-shell” and the “open-shell” states (Figure 3a and 3b). 1Σ+ is a higher lying singlet state where an additive linear combination of the “closed-shell” components is involved. (a) H N H (b) H H (c) H Figure 3. Representation of a) closed-shell components of 1Δ state of :NH b) open-shell and c) one of the spin components of the lowest triplet state (adapted from ref. 9) 5 Depending on the nature of the R group replacing the hydrogen atom, the degeneracy of the 1Δ components can be lifted making either the 1A' or the 1A" the lowest singlet states.
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